1. Field of the Invention
The present invention relates to a semiconductor device. In particular, the present invention relates to a structure of a pixel in an active matrix display which includes a light-emitting element and is manufactured using a semiconductor device. Moreover, the present invention relates to a display device equipped with a semiconductor device, and electronic equipment equipped with the display device.
A semiconductor device herein described indicates any device which can function by using a semiconductor characteristic.
2. Description of the Related Art
In recent years, demand for thin displays mainly applied to TVs, PC monitors, mobile terminals, and the like has increased rapidly and further development has been promoted. The thin displays include a liquid crystal display device (LCD) and a display device equipped with a light-emitting element. In particular, an active matrix display using a light-emitting element is expected as a next-generation display because of its features of high response speed, wide viewing angle, and the like in addition to advantages of a conventional LCD such as thinness, lightness in weight, and high image quality.
In an active matrix display using a light-emitting element, the most basic pixel structure is a structure shown in FIG. 24A (e.g., see FIGS. 19, 20A and 20B in Japanese Published Patent Application No. 2004-004910). In FIG. 24A, the pixel includes a driving transistor 2402 for controlling current supply to a light-emitting element 2404, a switching transistor 2401 for taking a potential of a data line 2406 into a gate (hereinafter also called a “node G”) of the driving transistor 2402 by a scan line 2405, and a holding capacitor 2403 for holding the potential of the node G
In FIG. 24A, the active matrix display including the light-emitting element 2404 can be driven by either an analog driving method or a digital driving method. In the analog driving method, an analog value is supplied to the gate of the driving transistor 2402 and the analog value is changed continuously, thereby expressing grayscale. In the digital driving method, a digital value is supplied to the gate of the driving transistor 2402: in the digital driving method, there is a digital time grayscale method in which one frame period is divided into a plurality of subframes and a light-emission period is controlled, thereby expressing grayscale. The digital driving method is advantageous in that it is hard to be affected by variation in transistors as compared to the analog driving method.
A specific example of a potential relation and operation timing when driving the pixel in FIG. 24A is shown in FIG. 24B, and the operation is described. At this time, the light-emitting element 2404 is driven by the digital driving method. As shown in FIG. 24B, the potential of the data line 2406 is taken into the node G when the potential of the scan line 2405 is a potential (a High potential, here) at which the driving transistor 2402 is turned on in the pixel structure shown in FIG. 24A.
In FIG. 24A, since the switching transistor 2401 is an N-channel transistor and the driving transistor 2402 is a P-channel transistor, the switching transistor 2401 is turned on and the potential of the data line 2406 is taken into the node G when the potential of the scan line 2405 is High. Respective potentials are set such that the light-emitting element 2404 emits light by taking a Low potential of the data line 2406 whereas does not emit light by taking a High potential of the data line 2406 into the node G.
As a specific example of the respective potentials, in FIG. 24A, the potential of a counter electrode of the light-emitting element 2404 is set to be GND (hereinafter 0 V), the potential of a current supply line 2407 is set to be 7 V, a High potential of the data line 2406 is set to be 7 V and a Low potential thereof is set to be 0 V, and a High potential of the scan line 2405 is set to be 10 V and a Low potential thereof is set to be 0 V.
Potential change of the wires is described using FIG. 24C. In a period during which the scan line 2405 has a potential of 10 V, the switching transistor 2401 is turned on and the potential of the data line 2406 is taken into the node G. By taking a potential of 0 V into the node G, a Vgs (a gate-source voltage) of 7 V is applied to the driving transistor 2402, thereby the driving transistor 2402 operates in the linear region enough. At this time, a voltage of about 7 V is applied to the light-emitting element 2404, and a current flows depending on resistance of the light-emitting element 2404 so that light emission is performed. On the other hand, by taking a potential of 7 V into the node G, the driving transistor 2402 is turned off because the Vgs thereof becomes 0 V, thereby the light-emitting element 2404 does not emit light. The potential of the node G is held by the holding capacitor 2403 until the potential of the scan line 2405 becomes High again.
In the example described using FIG. 24A, the potential of the node G is either the High potential or the Low potential of the data line 2406. The High potential of the data line 2406 is generally set to be the same as or higher than the potential of the current supply line 2407; therefore, when the voltage to be applied to the light-emitting element 2404, that is, the potential of the current supply line 2407 is increased, the voltage of the data line 2406 is also required to be increased.
In the digital driving method, selection pulses are outputted from a scan line driver circuit sequentially to rows of the scan line 2405, and data signals are outputted from a data line driver circuit at the same time to columns of the data line 2406 in accordance with the selection pulses.
Power consumption of a buffer portion in the data line driver circuit for charging/discharging the data line 2406 is dominant in power consumption of the driver circuits of the digital-driving display device. Power consumption P is generally calculated from the following formula (1), where F is frequency, C is capacitance, and V is voltage;P=FCV2 (F: frequency, C: capacitance, V: voltage)  (1)
It is therefore found from the formula (1) that decreasing the voltage of the data line 2406 to be set is effective to reduce the power consumption.